Electrical distribution systems are an essential part of the electrical power system. In order to transfer power from the place of generation to the place of use, some type of transmission network must be utilised. A large transmission system, of the type designed to transfer large amounts of energy to a large number of users, involves a system of interconnected power transmission lines originating at the electrical power generating stations (or power plants), which are typically dispersed over a wide area, with the ultimate purpose of the transmission lines being to bring the electrical power necessary for industrial, residential and commercial use to its point of utilisation.
In North America, the transmission of electrical power is largely in the form of a three phase alternating current (although there is some use of High Voltage DC, or HVDC, for transmission of large quantities of power over considerable distances). Power generating stations are typically located at some distance from the point of utilisation. For example, in Canada the James Bay Hydro Electric project, which has five major power stations producing a total of around 15,000 MW of power, is located in remote north west corner of the province of Quebec. The transmission lines, which are used to supply major urban centres in the south, extend over many hundreds of miles. The transmission lines, which are suspended from large steel towers, transfer three phase power at 60 cycles per second and an extra high voltage of 735 kV.
Closer to the points of utilisation, extra high voltage transmission lines are typically terminated by transmission substations where a reduction in voltage occurs for distribution to other points in the system through a medium voltage (MV) distribution grid. These MV transmission lines are typically referred to as the primary distribution network. Additionally, further voltage reductions for commercial and residential customers can take place at distribution substations, which connect to the primary distribution network.
Utility transmission and distribution systems link the power generating stations with end users through a network of power lines and associated components. In North America, typically the transmission portion of the system is designated as operating at 69 kilovolts (kV) and above, while the distribution portion operates between 110 volts and 46 kV.
Distribution networks are connected to the transmission portion of the system via transmission lines and substations. Similar to the transmission substations, distribution substations provide the link between a higher voltage transmission lines and a lower voltage transmission lines. The function of a distribution substation is to receive electrical power from the high voltage transmission and convert it to voltage levels suitable for industrial, commercial or residential use. Distribution substations are provided at various points throughout an electrical supply infrastructure.
The prior art reveals distribution substations engineered to provide power for a large number of users. These substations in many cases terminate the transmission line and as a result must maintain a high level of service. Consequently, they are designed with a large amount of redundancy in terms of transformers, disconnect switches, circuit breakers, bus bars and the like in order to provide continued operation under failure or high load. This in turn makes them large, resulting in a distribution substation having many components which is spread over a large area. Additionally, such prior art distribution substations are typically located at some distance to the high voltage transmission lines thus requiring the provision of an additional high voltage line between the transmission lines and the distribution substation. This gives rise to additional costs for erecting towers and running cabling as well as those related to the acquisition of land or grant of right of ways in order to ensure the unobstructed passage of these conductors.
In rural service areas the concentration of users is low and costs related to deploying a conventional substation are prohibitive. As a result, in many cases a power utility will not be able to generate an adequate return on the large investment necessary to bring a conventional distribution substation on line. On the other hand, rural users are typically satisfied with a lower quality of service and are willing to suffer some power outages if this means they will have access to electrical power at a reasonable price. Additionally, high voltage transmission lines on their path from power source to major urban centres typically traverse many rural areas to which they do not supply electricity.
In order to address the above drawbacks, the prior art reveals relatively small substations which can be located close to or underneath extra high voltage transmission lines. These non-conventional substations tap directly into the over head transmission lines using tapping through high voltage connectors as known in the art and do not otherwise interrupt the flow of power along the transmission line. Additionally, as the non-conventional substations provide a single transformer, no bus bar, which is used to interconnect transformers, is required. Also, only a single disconnect switch and circuit breaker is required. As a simple and cheaper alternative the circuit breaker can be replaced by a disconnect switch (or a load break switch) and set of fuses, significantly reducing the amount of equipment necessary to ensure operation of the non-conventional substation.
One drawback of the above non-conventional substations is that they do not provide any redundancy in the case of failure of one of the components of the substation or overload.